Found this cool micro UAV (DelFly Explorer) from MAV lab at TU delft. Apparently this is the first micro flapping wing UAV that is capable of autonomous flight thanks to the 9g stereo vision system. There are others that are smaller but cant fly on thier own! Other sensors include a pressure sensor for altitude. All of this comes in at just 20g and is capable of 9min of flight! The autopilot is able to perform auto take-off to a chosen altitude, and fly voiding obstacles (using stereo vision system). I dont think it will handle outdoor situations but still effective and very cool for indoor flight!
The key sensor of the DelFly Explorer is the stereo vision system. It weighs just 4g and is able to run an optimised stereo vision algorithm with other vision based algorithms (I am guessing optic flow). This vision system is coupled with the flight control system to perform short term path planning to avoid any obstacles. I would like to find out abit more about the actual processor they are using. But I guess you can find more details in thier pblication tittled : Autonomous Flight of a 20-gram Flapping Wing MAV with a 4-gram Onboard Stereo Vision System.
You can see more details of the DelFly Explorer and other MAV's they are working on at the MAV Lab website